With the rapid economic development and the increasingly strict environmental regulations, in the next ten years, China's demand for the clean energy of the natural gas will grow explosively, but the production of natural gas, though have increased, will still well lower than the increasing trend of its demand, so the contradiction between supply and demand has become increasingly prominent and the supply gap is increasing year by year. In view that China's energy source state is characterized in that coal is abundant, petroleum and natural gas are insufficient, so, an energy consumption structure that coal will be the major energy source will not change in the short term. According to the developing trend of clean coal technique and worldwide low-carbon economy, converting coal to natural gas, which is the best fuel among the fossil energy, is suitable for Chinese state and is a shortcut to eliminate energy crisis and ensure the energy safety.
Now, the process for preparing methane from coal can classified as indirect methanation and direct methanation. Indirect methanation is also called coal methanation process in two steps, wherein the first step is preparing syngas by gasifying coal, and the second step is preparing methane from the syngas (i.e., the purified coal gas in which H2/CO ratio having been adjusted). The direct methanation of coal refers to a process in which the coal is directly converted to the methane-rich gas product under a certain temperature and pressure. In this direct methanation process, the coal gasification operation and methanation operation, are not two separate operation.
FIG. 1 and FIG. 2 represent two typical process of current indirect methanation. FIG. 1 represents a process using a methanation catalyst which is not tolerant to sulfur, wherein coal is firstly gasified in a gasifier to produce syngas (its main components is CO and H2), then the syngas is subjected to primary purification procedure to remove dust, to cool down and to remove tar, then the syngas is coarsely desulfurized and finely desulfurized to remove the sulfide such as H2S, COS contained therein until the sulfur content in the desulfurized syngas is below 0.1 ppm, so as not to poison the methanation catalyst, then the C/H ratio of the syngas is adjusted by CO shift reaction (CO+H2O→CO2+H2) to meet the catalyst's requirement, then the syngas is introduced into circulation methanation reactor to be converted into methane product. The carbon contained in methane product is removed to get final gas product. FIG. 2 represents a process using sulfur-tolerant methanation catalyst. FIG. 2 differs from FIG. 1 in that the syngas is directly introduced into the methanation reactor to carry out sulfur-tolerant methanation reaction, instead of being desulfurized prior to being introduced into the methanation reactor. Then the reacted gas is subjected to subsequent procedures such as desulfurization and decarbonization to obtain final gas product. In above process for preparing methane from coal, the coal is must converted to syngas firstly, then the syngas is pretreated to remove dust and to cool down to meet the requirement of the catalyst in the subsequent methanation reactor, so the process flowchart is complex and the energy cost of the system is big. Furthermore, the methanation reaction, which is a strong exothermic reaction, tends to make the temperature of catalyst in the reactor run away and to deactivate the catalyst and shorten the life of catalyst. So, how to effectively remove the heat from the reactor is a problem to design the reactor.
Exxon Corporation in USA has carried out much experimental studies to the process for preparing methane from coal by one-step process. U.S. Pat. No. 4,318,712 discloses a whole process chart for the direct methanation of coal, see FIG. 3, in which the coal is premixed with catalyst and then is introduced into a gasifier. The superheated steam is used not only as a gasifying agent but also as a heat source to maintain the reaction temperature in the gasifier at about 700° C. The temperature of the superheated steam is 850° C., and the reaction pressure of the gasifier is 3.5 MPa. Coal reacts with superheated steam under the action of catalyst and direct produced methane-rich gas product, as shown in FIG. 3.
GPE Corporation in USA has carried out further study on the base of the technique of EXXON Corporation. Patent US20070000177A1 also a process for preparing methane from coal, wherein the catalyst is alkali metal carbonate or alkali metal hydroxide, the gasify agent is steam. In addition to adding effective methanation catalyst, the main technical features of this patent include adding calcium oxide to the coal powders to absorb the carbon dioxide produced during the reaction, so as to increase the methane content further.
The shortages of above process are: adding catalyst which promotes the generation of methane, but the high temperature is not favor of the generation of methane, so the reaction temperature is controlled at about 700° C., the reaction rate is slow and the carbon conversion is low, the reaction temperature is hard to maintain if heat is not provided by an external heat supplying system. Moreover, these techniques are still in study phase.
U.S. Pat. No. 4,077,778 proposes a process for catalytically gasifying coal by using multiple stages fluidized bed. This process eliminate the shortage of previous process and let the gasification more effective by making good use of the feeding carbon resource and increasing carbon conversion. The operating gas velocity of the primary fluidized bed is relative high to entrain some carbon particles into secondary fluidized bed, where the gasification reaction is carried out at a relative low gas velocity. In this way the residence time of the solid is increased, so the carbon conversion is maximized. The multiple stages gasification can increase the carbon utilization rate from 70-85% to above 95%, compared with single stage gasification. This process for catalytically gasifying coal by using multiple stages fluidized bed uses several fluidized bed reactors, so the invest for the equipments is high and the operation is relatively complicated.
The invention made a modification to the traditional process for preparing methane from coal. The invention integrates three operation process, i.e., the preparation of syngas from coal, the catalytically methanation of coal and the methanation of syngas, into a single reactor, so as to make good use of the energy.